Here is a possible alternative to your plan that will work. You could keep the boat in Riverside, NJ and still use it on the Delaware. Pay the NJ 5% tax when you buy the boat. Then when you move to MD, you can pay the extra 1% and register with them.

I actually did this back in 1997 and I had no problems with taxes. This is the first time the state of PA came after me for a "use fee". I bought another boat in 2002 in MD and paid taxes on it there and they came after me again. I think they scour the Coast Guard data base of Documented vessels for owners living in PA.